1. Field of the Invention
This invention relates to removing frozen material from a cryogenic separation unit without shutting down that unit. In particular, this invention relates to the production of olefins by thermal cracking a hydrocarbon containing feed and separating from said cracked feed a high purity molecular to hydrogen stream by use of at least one cryogenic unit (hereinafter sometimes “cold box” or “unit”). More particularly, this invention relates to thawing and removing frozen material in said unit without shutting down the olefin production plant as a whole, with minimal loss of production from said plant, and with favorable environmental impact results.
2. Description of the Prior Art
Although this invention has broader application, it will, for the sake of clarity and brevity, be described in its application to an olefin production plant.
Thermal cracking is a petrochemical process that is widely used to produce olefins such as ethylene, propylene, butenes, and butadiene, and aromatics such as benzene, toluene, and xylenes.
Basically, the hydrocarbon containing feed, for example, naphtha, vacuum gas oil, natural gas condensate, or other liquid hydrocarbons at ambient temperature and pressure, is mixed with steam which serves as a diluent to keep hydrocarbon molecules separated, and the steam/hydrocarbon mixture is subjected to an elevated temperature of from about 1400° F. to about 1500° F. at from about 15 to about 25 psig in a pyrolysis furnace (steam cracker). The cracked feed contains gaseous hydrocarbons of great variety, e.g., from one to thirty-five carbon atoms per molecule. Such hydrocarbons can be saturated, monounsaturated, and polyunsaturated, and can be aliphatic, and/or aromatic. The cracked feed also contains significant amounts of molecular hydrogen formed by the well-known free radical mechanism as saturated molecules such as ethane are converted during cracking into unsaturated molecules such as ethylene thereby freeing hydrogen radicals. Hydrogen radicals thus formed during cracking collide with one another to form a stable hydrogen molecule. Thus, the original feed, even if it initially contained molecular hydrogen, hereinafter “hydrogen” unless otherwise stated, is substantially enhanced in hydrogen content. This enhanced hydrogen content in the cracked feed is desirably separated from the hydrocarbon products also present.
Thus, thermal (steam) cracking is a noncatalytic process that employs diluent steam which does not participate chemically in the process as it does in steam reforming. The pyrolysis (cracked) product can contain, for example, based on the total weight (“wt.”) of the product, about 2 wt. % hydrogen, about 10 wt. % methane, about 25 wt. % ethylene, and about 17 wt. % propylene, with the remainder being a very wide variety of other hydrocarbon molecules from four carbon atoms per molecule down to a tar like material that contains hydrocarbon molecules having from 30 to 35 carbon atoms per molecule. But this is not all that is contained in the cracked feed product.
Because the thermal cracking process is so robust, it can handle a very wide chemical range of feedstock compositions. This allows for broad feedstock specifications for cracking operations. Because such feedstock specifications are not as tight as other processing units, for example, a hydrotreater, cracking processes are more likely to have chemical impurities in their feedstocks. For example, sulfur containing compounds and nitrogen bearing compounds like ethanolamine can often be present in thermal cracking feedstocks. The sulfur bearing compounds pose no particular problem in processes normally employed on the product effluent of the cracking process. For example, conventional caustic washing, which often is employed in such subsequent processes, readily removes sulfur compounds. On the other hand, nitrogen bearing compounds often break down under thermal cracking conditions to yield materials such as ammonia and various nitrogen oxides. Unfortunately, these materials separate out in subsequent processing by freezing solid in cryogenic units.
In order to separate a high purity hydrogen stream product, e.g., at least about 95 wt. % hydrogen, from said cracked feed, cryogenic units that can operate at temperatures below −240° F. are employed. Such units essentially liquefy the cracked feed except for hydrogen and some methane, and sometimes some ethylene and propylene. These cryogenic units are normally multiple pass heat exchangers that are well known in the art and contain a plurality of individual heat exchange sections that are physically close to one another if not contiguous, but operate independently of one another. Different fluid streams at differing temperatures flow through discrete and physically separate sections of a single unit at the same time, the result being that after the cracked feed and/or various fractions thereof have passed through one or more sections of said unit at one or more temperatures varying from about 80° F. down to about −270° F., the desired high purity hydrogen stream is achieved and ready for separation from said plant as a valuable product thereof. This hydrogen product is rich in hydrogen, but, depending upon various factors such as its operating efficiency, can still contain small amounts of light hydrocarbon, e.g., methane. The less hydrocarbon in the hydrogen stream product the better for obvious commercial reasons.
However, multiple pass heat exchangers (hereinafter referred to from time to time as a “unit”), with their high heat exchange capacity, have very small internal clearances, e.g., 2 millimeters, for the various streams passing through various sections thereof to be cooled or heated as the case may be.
Since such units operate at very low temperatures with very narrow clearances, it is vital to keep the clearances free of obstructions in order that the unit operates at its peak efficiency. Should the operating efficiency of such a unit degrade, even to a slight extent, the purity of the various product streams of the olefin plant can deteriorate. For example, should one or more sections in the unit experience the formation of frozen material (rime) in their internal passageways (cryogenic fouling), the result can be the invasion of increasing amounts of undesired light hydrocarbon, even ethylene and propylene, into the high purity hydrogen product of the plant. This decreases the commercial value of that product.
The various sections of the unit operate at different temperatures depending upon what stream is passing through a particular section. For example, one section of a unit could be operating at an Inlet temperature of −35° F. and outlet temperature of −50° F., while a nearby or contiguous section is operating at an inlet temperature of −147° F. and outlet temperature of −103° F., while yet another nearby or contiguous section in the unit is operating at an inlet temperature of −147° F. and an outlet temperature of −204° F. Thus, it can be seen that a specific stream containing materials having various freeze points may pass through one or more sections of the unit without forming a rime, but then may encounter a separate section that is operating at a lower temperature than the other section(s), and rime could form in that section thus adversely affecting the overall heat transfer efficiency of the unit and the quality of its various outflow streams.
As aforesaid, nitrogen containing compounds can be present in streams that are passing through the various sections of the unit. Unfortunately, a number of these compounds have freezing points (temperatures) within the operating temperature range of the overall unit. For example, nitrogen dioxide freezes at 12° F., ammonia freezes at −108° F., nitrogen trioxide freezes at −152° F., and nitric oxide freezes at −264° F. Accordingly, nitrogen containing compounds are a prevalent source of cryogenic fouling.
Heretofore, when a unit has experienced cryogenic fouling, the entire olefin plant has been shut down, and the entire unit defrosted to melt the rime. This can require disposal, e.g., flaring, of the entire feed to the plant.
It is desirable to keep the plant and unit operating while thawing and removing the offending rime from the section(s) of the unit that is afflicted with cryogenic fouling. This approach minimizes the adverse productivity impact on the plant. In addition, this approach minimizes possible environmental impact that may be necessary during shutdown for thawing.
By this invention, a unit is derimed in only the section(s) experiencing cryogenic rime fouling while leaving the plant and other sections of the unit operating at or at least near their normal conditions.
Thus, by the practice of this invention, a unit is derimed in a fraction of the time it would take to thaw the entire unit, with minimal loss of plant production, and with favorable environmental results.